Flow injection analysis (FIA) is an important chemical analysis method which is described, for example, in the book Flow Injection Analysis by Ruzicka and Hansen, 1981, Wiley Publishers, New York, and by the many publications and patents naming Hansen and Ruzicka as authors and inventors, and in the extensive publications and patents of others. There are many variations of FIA, but basically a carrier liquid is flowed sequentially through a sample injector, an in-line mixer and a detector. A preselected volume of sample is injected into the carrier stream by the injector. The in-line mixer is usually simply a coil of tubing so that the injected sample is dispensed into the carrier in a controlled manner. The carrier is usually premixed with a reagent which can react with a component of interest of the sample to produce a reaction product that is sensed as a "peak" by the detector. The detector is usually a photometer based system having a flow cell through which light is shown to detect the reaction product. Chemical sensing probes such as pH electrodes and ion specific electrodes have also been used to detect the reaction product: by immersing the probe in a stirred chamber into which the carrier stream from the mixer is fed; by channeling the carrier stream from the mixer across an active portion of the probe; and by flowing a film of the carrier stream from the mixer across an active portion of the probe.
If the reaction product is a precipitate, then FIA is often unsuccessful because the precipitate tends to clog the detector portion of an FIA system with deposits of the precipitate. This problem is especially apparent with extended use. As a solution to this problem, some workers periodically or continuously added precipitate dissolving chemicals to the carrier stream. However, some precipitates are difficult to dissolve. As a rule, precipitation reactions in FIA have not been widely used because of the above-mentioned problems and it would be an advance for FIA if a detection flow cell was designed so as to allow extended use for precipitation reactions without the need for precipitate dissolving chemicals.